Overview
This lecture introduces dihybrid crosses, explaining their principles, how to perform them, and the resulting phenotypic ratios when crossing different parental genotypes.
Monohybrid vs. Dihybrid Crosses
- Monohybrid crosses involve one trait; dihybrid crosses involve two traits at the same time.
- Dihybrid crosses examine the inheritance of two different genes, such as fur color and eye color in cats.
Mendel's Laws in Dihybrid Crosses
- The Law of Independent Assortment states that genes for different traits are inherited independently.
- The Law of Segregation ensures each gamete receives only one allele of each gene.
Setting Up a Dihybrid Cross
- Choose clear letters for each allele to distinguish dominant and recessive traits.
- Use a key (e.g., F for dark brown fur, f for cream fur; E for green eyes, e for blue eyes).
- Always create eight gametes by combining one allele from each gene for each parent.
Punnett Squares and Pure Breeding
- Arrange gametes from each parent along the sides of a Punnett square, keeping individuals separate.
- Pure breeding or true breeding means parents are homozygous for both traits (either dominant or recessive).
- Crossing purebreds (e.g., RRYY x rryy) produces uniformly heterozygous offspring (RrYy) with one phenotype.
Crossing Heterozygous Individuals
- Crossing two heterozygous individuals (RrYy x RrYy) requires careful gamete formation.
- The phenotypic ratio resulting from this cross is always 9:3:3:1 (9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green).
- Identify phenotypes by looking for the presence of dominant or recessive alleles in each combination.
Genotype vs. Phenotype Ratios
- Phenotypic ratio describes physical traits seen in offspring (e.g., color, shape).
- Genotypic ratio describes the combinations of alleles present in offspring.
Key Terms & Definitions
- Dihybrid Cross — Genetic cross involving two traits.
- Law of Independent Assortment — Genes for different traits are inherited independently.
- Law of Segregation — Gametes receive only one allele from each gene.
- Allele — Variation of a gene; for example, R or r for seed shape.
- Genotype — The genetic makeup (allele combination) of an organism.
- Phenotype — The observable trait of an organism.
- Pure Breeding (True Breeding) — Organisms homozygous for all traits considered.
Action Items / Next Steps
- Practice setting up dihybrid Punnett squares for both purebred and heterozygous crosses.
- Memorize the 9:3:3:1 phenotypic ratio for heterozygous dihybrid crosses.
- Review the definitions and make sure you can distinguish genotype from phenotype.